1. Field of the Invention
This invention is directed to decoder circuits for convolutional codes, in general, and to a circuit which uses the Viterbi-type techniques, in particular.
2. Prior Art
Since the early 1950's many methods of coding have been proposed for error control in digital communication systems. One of the most widely accepted and implemented systems consists of convolutional codes decoded by circuits employing the Viterbi algorithm. This combination, using soft decisions, approaches the optimal decoding strategy (maximum likelihood sequence decoding) for channels where the interference is additive-white-Gaussian-noise (AWGN). The AWGN channel is a good model for both benign and hostile environments where errors effect each bit independently. For channels where errors affect several contiguous bits, as in fading or partial time or partial band jamming, the above coding/decoding system can still be employed with considerable success (approaching optimum performance) by "randomizing" the effect of the channel errors by a process known as interleaving.
In the past, Viterbi-type decoders of constraint length 7 or more have required substantial quantities of circuitry, components, power utilization, etc. Decoders of constraint 3 and 5 have been implemented in VLSI but these devices do not provide sufficient coding gain (communication system performance improvement) for most applications. In some instances this is of little or no consequence, as for example in a stationary ground-to-ground technique. However, in an airborne or space capsule environment, the size, weight and power consumption all become critical. Consequently, it is highly desirable to have a decoding system with minimum size, weight and power requirements. The instant invention is a new and unique apparatus for reducing the operating parameters of weight, size and power consumption while increasing, at least in a relative sense, the operating characteristics of the decoder.